Narrowband wireless communication systems require the transmitted signal spectrum to be compact in order to meet strict adjacent channel interference specifications. The pulse shape filters that are utilized to achieve such specifications generally possess relatively slow decaying impulse responses. These impulse responses generate substantial Inter Symbol Interference (ISI). In linear modulation schemes such as Quadrature Amplitude Modulation (QAM), the combination of this ISI and certain transmitted symbol sequences can create large local peaks in the transmitted signal envelope. Large local peaks equate to a large peak-to-average power ratio which in turn requires the transmitter power amplifier to operate at low efficiency.
Conventional 16-level QAM signaling (i.e. square signal constellation w/root raised cosine rolloff pulse shaping) has a peak-to-average power ratio of 8.24 dB and 6.6 dB for rolloff values of 0.2 and 0.35, respectively. Considering the theoretical efficiency limit of a Class AB amplifier these ratios would require the transmitter amplifier to operate at an efficiency of only 18 and 24%, respectively. This compares rather poorly with the 65% efficiency that can be attained with constant envelope modulation. In general, in one dimensional digital communication systems, the transmitted waveform is formed by adding time-shifted versions of a basic pulse shape. The amplitude of the individual pulses is adjusted according to the data being sent (e.g. binary phase shift keyed). Similarly, multi-dimensional digital communication systems (e.g. QAM) construct the transmitted waveform from multiple pulse streams that are generated according to the data. Most digital communication systems utilize a pulse shape that spans several symbol intervals to minimize the bandwidth of the transmitted waveform and thereby secure that the transmitted waveform does not interfere with other systems operating in a nearby (frequency) channel. The result of this pulse shaping is that the pulse associated with one data symbol will overlap pulses associated with adjacent data symbols. Certain data sequences will cause these overlapping pulses to add constructively and produce large peaks in the envelope of the transmitted waveform, while other data sequences will cause these overlapping pulses to cancel one another and produce small envelope values of the transmitted waveform. Amplifiers that are used to boost the power of the transmitted signal just prior to transmission work best when the signal remains at a fairly constant level. The severity of the signal envelope fluctuations is often quantified by the ratio of the peak-to-average signal power or peak-to-average power ratio. Large peaks in the transmitted signal lead to inefficient usage of the power amplifier which in turns wastes precious battery life.
Battery operated communication devices employ a variety of techniques to save battery energy in order to prolong the operating life of the battery. Increasing the efficiency of power amplifiers is just one technique. Another scheme by which battery energy may be saved is the use of a power-efficient modulation technique. Various modulation techniques have different associated peak-to-average power ratios. In general, it is highly desirable to have a peak-to-average ratio as close to zero dB as possible (e.g. FM). This minimizes the demand placed upon the amplifier to handle peak powers that are significantly larger than the average power. Thus allowing the amplifier to operate at a point of higher efficiency, and thereby reduce the energy drain on the battery. However, many existing modulation formats generate relatively high peak-to-average power ratios. Two commonly used modulation formats are Phase Shift Keying (PSK) and QAM. The former uses a signal constellation where all data symbols have the same magnitude while the latter varies both the phase and magnitude of the individual data symbols. Binary signaling is a special case of PSK (i.e. BPSK). In both modulation formats, the peak-to-average ratio depends upon the pulse shape used.
Quadrature Amplitude Modulation (QAM) utilizes both the phase and amplitude of a carrier to transmit information and hence has the potential to generate a higher peak-to-average power ratio. Indeed, experiments have demonstrated that, for example, a sixteen symbol PSK constellation enjoys a 3-4 dB advantage in peak-to-average power ratio over a 16 QAM signal. However, this gain in efficiency improvement is accompanied with a 4 dB loss in sensitivity. Due to this loss of sensitivity, many system designers prefer to use the QAM modulation format despite its degraded peak-to-average power ratio.
Referring to FIG. 1, a communication device is shown as is presently available. FIG. 2 shows a phase and magnitude trajectory of a complex baseband 8 PSK signal. In other words, this figure represents the transition from one data symbol to the next as the generated data changes state. A pulse shape filter that is used to limit the sideband noise produces undesirable overshoot as shown by reference 202. This overshoot 202 contributes to an increase in peak power which in turn increases the peak-to-average power ratio. This increase in the peak-to-average power ratio forces a designer to design an amplifier that can tolerate the maximum peak power which in turn renders the power amplifier more expensive to produce. In addition, the increase in peak-to-average ratio reduces the power efficiency of the power amplifier, thereby wasting battery life.
In the design of portable communication devices, the aim of a designer is to utilize efficient components at the lowest possible price. Power amplifiers have traditionally been some of the most expensive components of a communication device and have often resisted attempts aimed at lowering their cost. One parameter that is directly related to the cost of amplifiers is the peak-to-average power ratio. This is because the designer is forced to employ an amplifier that can handle peak powers significantly larger than the average power. It has therefore been the goal of designers to reduce peak-to-average power ratios as much as possible without degrading other performance parameters. There is therefore a need for a modulation scheme that would have minimum peak-to-average power ratio without suffering other performance degradation.